A typical cause of damage to axle gears is damaging by fatigue, which occurs under repeated efforts. This phenomenon is induced by repeated rolling stresses under strong loads.
This is expressed by pitting (surface fatigue) or spalling (deep fatigue) phenomena. Spalling occurs after a long aging time, preceding visible deteriorations. The mechanisms are badly known, but the phenomenon starts by initiation of cracks at a certain depth under the surface, these cracks propagate, and when cracks normal to the surface are created, spalls are suddenly detached therefrom.
The spalling phenomenon is particularly sensitive on axles of heavy trucks, where the gears are subject to very strong loads and to very long periods of use.
Preventing this phenomenon goes through a reduction of the contact stresses by means of a suitable geometry of the parts, and through reduction of friction, while avoiding adhesion.
The lubricant is involved in this prevention process because of its viscosity and the physico-chemical reactivity of its additives.
Sulfur additives, as well as phosphorous and sulfur-phosphorous additives are widely used in the formulations of oil for axles and gear boxes, as anti-wear and extreme pressure additives. They protect frictional parts under a strong load by forming a film adsorbed on their surface.
Sulfur additives, forming a tribo-film of iron sulfide with a low shear resistance, easily detachable, notably allow prevention of seizing-up, a phenomenon which occurs right from the first stages of use, when the resistance of the interface exceeds that of the underlying material.
The additives present in the lubricant may also have an either positive or negative impact, on the propagation of cracks inside the parts and therefore on the spalling phenomenon.
There is therefore a need for increasing the life time of frictional parts, notably of axle gears, while limiting the occurrence of the spalling phenomenon.